Project Summary/Abstract The distributed brain network of the hippocampus supports memory and related cognitive abilities. Disruptions of this network occur in many neurological disorders such as epilepsy, brain injury, and neurodegenerative disease. Brain stimulation targeting the human hippocampal network can produce long-lasting improvements of memory ability, with corresponding increases in brain-activity markers of network function. However, mechanisms for this beneficial network-level neuroplasticity caused by brain stimulation remain unknown. Mechanistic knowledge is essential to optimize how and where to stimulate the hippocampal network in order to maximize the resulting memory benefits. This project will investigate the cellular mechanisms for the effects of brain stimulation on the hippocampal network. We will capitalize on the property that activity of regions of the hippocampal network synchronize in the theta frequency band (5-8Hz) to test for mechanistic homology in the effects of stimulation on human versus rodent hippocampal networks. In humans undergoing neurosurgery for intractable epilepsy and in awake, behaving rodents, we predict that electrical stimulation will have greater effects on hippocampal network function when it is delivered with increasing levels of synchronization to the ongoing hippocampal theta activity rhythm. Thus, we will test whether the effects of manipulating the synchrony between brain stimulation and hippocampal theta activity are comparable in humans and rodents. The effects of stimulation will be assessed using measures of hippocampal network functional connectivity and paired-associate memory performance that can be performed similarly in both species. We will then conduct in vitro electrophysiology experiments in rodent brain slices obtained after stimulation in order to identify cellular mechanisms for the effects of stimulation. We predict that stimulation parameters that increase hippocampal network function will increase cellular excitability, as measured via the postburst afterhyperpolarization, of dorsal hippocampal CA1 pyramidal neurons. Viral manipulation of CREB expression, which is necessary for changes in excitability, will be used to causally test the role of dorsal hippocampal CA1 excitability in the effects of stimulation on hippocampal network function. These research objectives are in close alignment with the focus of RFA-NS-18-018 on establishing cellular mechanisms for the effects of brain stimulation on neuronal circuits. Findings will uniquely uncover cellular mechanisms by which brain stimulation beneficially impacts distributed brain networks and corresponding cognitive abilities. These mechanistic insights could propel brain-stimulation treatments for memory impairments caused by disruption of the hippocampal network.